The olfactory epithelium (OE) is unique in its ability to undergo regeneration of all cell types (both neurons and support cells) throughout mammalian life in response to both natural cell turnover, as well as environmental insult. Environmental injury can be easily induced in a laboratory setting through methyl bromide (MeBr) gas exposure, allowing for a controllable method to activate stem cells in vivo. The progenitor population responsible for regeneration has been identified as two classes of basal cells, globose basal cells (GBCs) and horizontal basal cells (HBCs). Both the ease of acquiring activated stem cells along with knowledge of the identity of the progenitor population, make the OE an ideal model in which to study regenerative medicine. In order to harness the potency of regenerative therapies, it is first necessary to dissect out the signaling pathways that govern stem cell differentiation. The olfactory epithelium contains an underlying mesenchymal compartment, the lamina propria (LP), which remains ill-defined. The importance of mesenchymal-epithelial communication is well-studied in other areas of stem cell biology, where factors secreted by mesenchymal cells signal via epithelial cells to drive differentiation. Understanding how this mesenchymal layer influences epithelial differentiation appears to be imperative in beginning to define signaling pathways that direct progenitor cells towards differentiation. The ErbB receptor family binding protein, Neuregulin1 (Nrg1), has been found to be secreted by LP cells and has been shown to be capable of inducing OE stem cell differentiation in a 3D culture model in vitro in our lab. The goal of this study is to understand he effects of Nrg1-induced differentiation. First, a 3D culture model designed to allow for sphere growth and stem cell differentiation when in the presence of appropriate growth factors, will be studied for the ability of Nrg1 to stimulate sphere growth. Spheres stimulated to grow with Nrg1 will be analyzed through both Multisizer quantification of sphere number and size and confocal microscopy of spheres immunostained with markers for different OE cell types. This will provide a determination of the efficacy of Nrg1 to induce differentiation. Secondly, a knock-out mouse for the Nrg1 receptor, ErbB4, will be used to study the effects of loss of Nrg1/ErbB4 signaling in vivo. These studies will focus on characterizing the knock-out phenotype using immunohistochemistry, and performing 3-D in vitro assays to determine the functional effect of ErbB4 loss on sphere growth and differentiation. Together, these studies will provide novel insight on the effects of a mesenchymal-secreted protein, Nrg1, on OE differentiation, and implicate a well-known developmental signaling pathway in OE differentiation.